1. Field of the Invention
The present invention relates to a pneumatic tire which can suppress a heel and toe wear while holding a hydroplaning resistance.
2. Description of the Related Art
In a studless tire which is used in a traveling on a snowy road, a cut called as a sipe is formed in a block on a tread, and a drivability on the snowy road is enhanced by an edge effect of the sipe. On the other hand, if a groove depth comes to a halving state due to a progress of a wear, it is hard to hold a studless performance. Accordingly, there is an actual condition that the tire cannot be used as a winter tire. Therefore, in order to extend a tire service life, there can be considered that a worn studless tire is utilized as a summer tire, however, it is important to keep a draining performance and improve an irregular wear resistance for that purpose.
In Japanese Unexamined Patent Publication No. 2003-159910, there is described a pneumatic tire in which blocks are comparted by a slit 21 and a chamfer 22 is formed in an edge of a tread surface thereof, as shown in FIG. 8. There is described that it is possible to improve a draining performance by an enlargement of a groove volume in a wide portion 23, and it is possible to ease the heel and toe wear by the chamfer 22. The heel and toe wear is one form of an irregular wear which is generated on the tire, and is generated by one of a trailing side and a leading side of a block being worn on ahead in comparison with the other side in correspondence to an input in a circumferential direction to the block by a tire rolling.
In Japanese Unexamined Patent Publication No. 2008-260423, there is described a pneumatic tire structured such that a sipe 25 shown in FIG. 9 is formed in a land portion of a tread, and a tread pattern is changed from an asymmetric shape to a point symmetric shape by an exposure of a cavity 26 in accordance with a progress of a wear. There is described that a heel and toe wear can be suppressed by changing an installation direction of the tire so as to invert a trailing side and a leading side after the change of the tread pattern mentioned above.
However, since a rigidity in one side which is widened by the wide portion 23 is lowered over a whole length in the slit 21 mentioned above, and a rigidity in the both sides which are widened by the cavity 26 is lowered over a whole length in the sipe 25 mentioned above, a rigidity of an end portion of the block tends to be reduced, and an effect of suppressing the heel and toe wear cannot be regarded as being sufficient. Further, since there is a fear of such a rigidity reduction, there is a case that it is actually hard to greatly widen the wide portion 23 and the cavity 26, and there has been a possibility for improving the hydroplaning resistance.